1. Field of the Invention
The present invention relates to a method for removing fission products in a spent electrolyte produced in a pyro-reprocessing process and an apparatus used therefor.
2. Description of the Related Art
A spent electrolyte (salt waste) produced from a pyro-reprocessing process using metal electrolysis contains various fission products, and is classified into a high-level radioactive waste. An electrolyte used in the pyro-reprocessing process includes LiCl—KCl as the medium. Fission products (FPs) mainly containing elements such as alkali metals, alkaline earth metals, and rare earth elements are dissolved into the spent electrolyte as chlorides. Then, from viewpoints of reduction in environmental impact, economical efficiency, and the like, the fission products in the spent electrolyte need to be separated for reduction in a volume of the high-level radioactive waste and for regeneration and reuse of the spent electrolyte.
A method for processing a spent electrolyte has been proposed as a technique of separation of the fission products and regeneration of the medium, including: a spent electrolyte regeneration step of reacting FP chlorides accumulated in the spent electrolyte with phosphates to convert the FP chlorides into FP phosphates, precipitating the FP phosphates, and regenerating the medium; a phosphate conversion step of reacting excessive salts produced by the spent electrolyte regeneration step with phosphoric acid to convert the excessive salts into phosphates; and a phosphate immobilization step for stabilizing the produced phosphates by filling iron phosphate glass with the produced phosphates (see Japanese Patent Laid-Open No. 2007-303934).
As a phosphate conversion material that precipitates and separates the fission products dissolved as chlorides, lithium phosphate (Li3PO4) or potassium phosphate (K3PO4) is suitable for its smaller influence given to the composition of the electrolyte. When lithium phosphate (Li3PO4) or potassium phosphate (K3PO4) is used, chlorides of lanthanoid series (Ln) are easily converted into phosphates so that phosphates are precipitated. However, chlorides of alkali metals (except Li) and alkaline earth metals form no precipitate irrespective of addition of, the conversion material.
A method for separating the precipitate is generally filtration. Various filtration apparatuses have conventionally existed. However, the filtration apparatuses are usually used at room temperature, and such filtration apparatuses hardly can separate a target substance in a melt with a high temperature. In addition, the target substance for separation includes not only FP elements that produce precipitates but also FP elements that produce no precipitate. Therefore, ordinary filtering materials cannot sufficiently remove all of these FP elements.
Then, the related prior art needs two separation and recovery methods for separating and removing the fission products dissolved into the spent electrolyte, and it has been examined that a liquid mainly containing a precipitate and a supernatant are selectively fed, the liquid containing the precipitate is separated by a filtration filter, and the supernatant is separated using a separation filter having ion exchange capacity/molecular sieving capacity (see Nihon Genshiryoku Gakkai “2008-nen Haru-no-nenkai” Youshishu D35). However, this method has had problems to be solved in practice such that operation is complicated, a dedicated worker must be disposed, and the configuration of the apparatus is complicated.